Normally, Scala symbols can be thought of as a unique identifier which will always be the same. Every symbol that is lexi-graphically identical refers to the exact same memory space. There's nothing else that's special about them from Scala's point of view.
However, Dispatch-Json pimps out symbols making them JSON property extractors. To see the code which is responsible for the pimping, check out the SymOp class and the rest of the JsonExtractor.scala code.
Let's write some code which solves the problem you are looking at and then analyze what's going on:
trait ExtUserProps extends UserProps with Js {
val friends_count = 'friends_count ! num
}
object ExtUser extends ExtUserProps with Js
val good_stuff = for {
item <- http(Status("username").timeline)
msg = Status.text(item)
user = Status.user(item)
screen_name = ExtUser.screen_name(user)
friend_count = ExtUser.friends_count(user)
} yield (screen_name, msg, friend_count)
The first thing that we're doing is extending the UserProps trait in the Dispatch-Twitter module to give it a friends_count
extractor and then defining a ExtUser
object which we can use to get access to that extractor. Because the ExtUserProps extends UserProps, which also extends Js, we get the method sym_add_operators
in scope which turns our symbol 'friends_count
into a SymOp case class. We then call the !
method on that SymOp which we then pass the Extractor num
to, which creates an Extractor that looks for a property "friends_count" on a JSON object and then parses it as a number before returning. Quite a bit going on there for such a small bit of code.
The next part of the program is just a for-comprehension that calls out to the Twitter timeline for a user and parses it into JsObjects which represent each status item, them we apply the Status.text
extractor to pull out the status message. Then we do the same to pull out the user. We then pull the screen_name and friend_count out of the user JsObject and finally we yield a Tuple3 back with all of the properties we were looking for. We're then left with a List[Tuple3[String,String,BigDecimal]] which you could then iterate on to print out or do whatever with.
I hope that clears some things up. The Dispatch library is very expressive but can be a little tough to wrap your head around as it uses a lot of Scala tricks which someone just learning Scala won't get right away. But keep plugging around and playing with, as well as looking at the tests and source code, and you'll see how to create powerful DSL's using Scala.